In today's point-to-point communications, trunked communication systems provide many advantages over conventional systems. The trunked communication systems provide efficient allocation of a limited number of communication channels to a large number of users. Additionally, many system features, such as channel privacy, user identification, etc., may easily be incorporated into the trunk communication systems.
In a trunked communication system, a communication resource controller and a plurality of communication units, such as portable and mobile two-way radios, communicate voice and data messages in a synchronous manner. The data messages, having a predetermined baud rate, are communicated on one or more control channels, and the voice messages are communicated on one or more voice channels. The communication resource controller, upon reception of an inbound channel request from one of the communication units, allocates a voice channel to that communication unit upon its availability. In conventional synchronous trunked communication systems, the communication resource controller receives only an inbound channel request during a predetermined access time interval. The access time interval equals duration of a predetermined number of data bits communicated between the communication unit and the communication resource controller. If an inbound channel request is received within the access time interval, a response is transmitted to the communication unit which then either indicates allocation of an available channel or indicates that all channels are busy.
The duration of the access time interval corresponds to the range within which a communication unit may properly communicate with the communication resource controller. For example, in a trunked communication system communicating data at a 3600 baud rate and having a 3 bit access time interval, the range of communication is approximately 75 miles. That is, a signal generated within a 75 mile range propagates during a 3 bit time interval before it reaches the communication resource controller. Therefore, the communication resource controller allows a 3 bit time interval for reception of an inbound channel request which translates into a communication range of 75 miles.
One of the problems associated with the 3 bit access time limitation is that a communication unit may be inhibited from accessing the communication system because the channel access request may reach the communication resource controller beyond the 3 bit time interval. This is especially true in mountainous communication sites where the radio frequency signals carrying the channel access request may be bounced off at many directions causing propagation delays of longer than 3 bit time interval. Also in ranges of longer than 75 miles, the communication unit may be inhibited from accessing the communication system even though its transmitter power output allows the communication unit to effectively communicate with the communication resource controller. Under these conditions the channel access transmissions from the communication unit may be ignored if the propagation delay exceeds the 3 bit time interval.
One prior art approach for solving the access problem discussed above causes the communication unit to transmit a channel access request and if a valid response is not received, the communication unit assumes that it is out of the communication range. Therefore, the communication unit advances transmission timing of the channel access in order to allow the channels access request to reach the communication resource controller within the 3 bit time interval. However, this method is iterative and time consuming. Furthermore, the decision to advance the transmission timing is based on an assumption which may not be true.
The access problem may also be overcome by utilizing asynchronous trunked communication systems. However, these systems have an inherently less efficient system throughput and do not offer many of the desirable features provided by synchronous communication systems.
It is, therefore, desired to extend the communication range of the trunked communication system, where the communication unit may access the communication resource controller when even when its distance prohibits access within the access time interval.